1. Field of the Invention
The present invention relates to an inverter control circuit and a control method thereof. More particularly, the present invention relates to an inverter control circuit with pulse width modulation (PWM), and a control method thereof.
2. Description of the Related Art
For a wide range of products from home electric appliances such as an air conditioner and a refrigerator, to industrial machines, motor controlling is performed. Particularly, in recent years, effective control for these electric appliances has become increasingly important in order to reduce power consumption.
For example, an air conditioner uses an inverter circuit to perform motor control in order to improve the accuracy of temperature control and reduce power consumption, and is required to achieve an optimum control of the motor. To be more specific, a pulse width modulation (PWM) drive system is widely used in which switch devices constituting an inverter circuit are turned ON/OFF to control power supply to a motor coil. A microcomputer, for example, is used as a control circuit for PWM driving and controls electric current supply to a motor coil by changing a pulse width for turning ON/OFF the switch devices. An A/D converter receives an analog signal (voltage) based on an electric current flowing through the inverter circuit and the motor, and converts the analog signal into digital data. Then, a central processing unit (CPU) controls the pulse width for the PWM driving so as to optimally drive the motor based on the above digital data.
In order to perform precise control based on an analog signal received by an A/D converter, the resolution of the A/D converter is important. Particularly, it is important that the A/D converter can achieve the high resolution irrespective of the level of the amplitude of an analog signal. In this regard, Japanese Patent Application Publication No. 2000-188548 discloses a technique employing an amplifier which amplifies an analog signal to be inputted into an A/D converter. In this technique, the gain of the amplifier is changed according to a control signal from a CPU, so that the A/D converter is enabled to surely achieve high resolution irrespective of the level of the amplitude of the analog signal.
For a data collection device described in Japanese Patent Application Publication No. 2000-188548, the CPU controls the switching of the connection of resistances in a second multiplexer, whereby the gain (the rate of amplification) is changed. However, when the device is designed to perform the switching control by use of the CPU, the CPU needs to execute processing for every switching. Accordingly, if the CPU performs other processing, a load on the CPU is increased, thus deteriorating the performance of the CPU. Further, when the CPU is executing other high-priority processing, waiting time occurs before actual switching, and this makes it difficult to perform switching at an optimum timing. Particularly, recently, reduction of power consumption of air conditioners, refrigerators and the like has been desired, and therefore improvement of efficiency of an inverter control motor has been desired. In such an application, when controlling an inverter, it is important to reduce a processing load of the CPU so as to reduce the deterioration of the performance thereof as much as possible.